Certain embodiments of the present invention relate to the display of video data. More particularly, certain embodiments relate to the vertical/temporal sampling and display of progressively scanned video data on an interlaced display device.
Vertical/temporal sampling and display of progressively scanned video data is discussed herein. It is contemplated that horizontal/spatial scaling of video data is handled independently of the vertical/temporal processing. scaling of video data is handled independently of the vertical/temporal processing.
“Artifacts” refer to visual blemishes in video that are not caused by the inherent sampling format and therefore are considered abnormal byproducts of video processing. “Top Field” refers to the field of video that starts at the first line in the displayed frame if each field of video is referenced to the physical start of the display. “Bottom Field”, refers to the field of video that starts at the second line in a displayed frame if each field of video is referenced to the physical start of display of the first line of the frame. “Even/odd lines” refers to the even lines numbered 0, 2, 4, etc. (i.e. Top field in an interlaced frame) and the odd lines numbered 1, 3, 5, etc. (i.e. Bottom field in an interlaced frame) when the lines of a frame are numbered starting from ‘0’ and a counter is incremented by one for each line. “Frames” refers to sets of video lines that constitute a coherent picture. “Frame-time” refers to the period of time used to sample or display all the lines in the frame. One frame-time of 1/30th of a second, for example, is the time interval between frames (i.e. between the starting times of successive frames). “Fields” refers to sets of video lines that are parts of frames of video wherein each field of a frame has about ½ the number of the total lines in the frame. “Field-time” refers to the time interval between two fields (i.e. between the starting times of successive fields).
Two types of video data, “progressive” and “interlaced” video data, are typically generated as frames of video. An interlaced video comprises two fields that are sampled at two different times; and each field comprises either the even or odd lines of the video data. For interlaced video, half the lines in the frame are displayed in one field-time and the other half of the lines are displayed in the next field-time. Each frame of video is displayed on a presentation device at a specific frame rate. For purposes of discussion herein, a 30 Frame/sec (“fps”) rate is used to indicate the display rate for frames while a 60 Field/sec rate is used to indicate the display rate for individual fields in an interlaced frame, where the interlaced frame (i.e., two fields) is displayed at 30 Frame/sec.
A progressive frame is usually sampled line by line after sampling starts at one time. A CIF (Common Image Format) frame (240 lines) is such an example. It may be displayed on a 525-line (about 480 active lines) or a 625-line (about 576 active lines) interlaced system, which is alternatively referred to as an “NTSC-display” or “PAL-display” respectively. While a representative frame rate of 30 fps may be utilized as provided above, other frame rates, including 25 fps, 29.97fps, 23.976 fps, etc., are contemplated.
In the interlaced video case, with top and bottom (or even and odd) fields, there is a vertical positional offset between the two fields. In the progressive video case, there are no fields.
In very general terms, the task of displaying progressive video content on interlaced presentation devices may involve changing the sampling rate and sampling grid. Not only are the lines in the original progressive frame displayed at field rates, but they are also displayed with a vertical offset between fields that are characteristic of interlaced displays.
Generally the progressive video data is meant to be displayed on a progressive presentation device. The video data is typically, but not always, displayed one line at a time starting from the top of the frame all the way to the bottom of the frame during one display time. If the lines in the progressive frame are numbered 0, 1, 2, 3, . . . and the data is sampled at a frame rate of 30 frm/sec, then all lines (0, 1, 2, 3, . . . ) may be displayed in succession in 1/30 second. However, this is not the case for typical CRT (cathode ray tube) designs, for example.
Generally the interlaced video is meant to be displayed on an interlaced presentation device as separate fields of video. If the lines in the interlaced video frame are numbered 0, 1, 2, 3, . . . then by convention the even-numbered lines belong to the top field and the odd-numbered lines belong to the bottom field. An interlaced frame of video implies that the fields are sampled at different times and are displayed on the presentation device at different times. For example, if an interlaced video frame is displayed in 1/30 second, the top field (lines 0, 2, 4, . . . ) is displayed in a 1/60 second time period and the bottom field (lines 1, 3, 5, . . . ) is typically displayed in the next 1/60 second period.
Displaying progressive video on a progressive presentation device generally results in no display artifacts. Similarly, there are no unexpected visual problems when interlaced video is displayed on interlaced presentation devices. Artifacts (both spatial and temporal) may be created when displaying progressive video on interlaced devices, or when displaying interlaced video on progressive devices.
Video lines in a progressive frame containing “n” lines may be denoted as p0, p1, p2, p3, . . . p(n−1) and video lines in an interlaced frame containing “m” lines are denoted as i0, i1, i2, i3, . . . i(m−1), where the top field comprises the even numbered lines i0, i2, i4, . . . i(m−2) and the bottom field comprises the odd numbered lines i1, i3, i5, . . . i (m−1).
Progressive content may be displayed on interlaced devices using progressive lines p0, p1, p2, p3, . . . p(n−1) in one field-time interval ( 1/60 sec), and the same lines p0, p1, p2, p3, . . . p(n−1) in a second field-time interval ( 1/60 sec) where “n” is the number of progressive video lines. If “n” is not equal to “m/2” (where m is the number of interlaced video lines), spatial (i.e., vertical) scaling is performed to match the progressive video lines to the interlaced display format. Since the lines of the interlaced fields have a different vertical position from one field-time interval to the next, the display of the progressive video lines p0, p1, p2, . . . for both fields causes a vertical shift that manifests itself as a display artifact that looks like vertical hopping. Vertical scaling may produce additional artifacts depending on the scaling ratios used.
Progressive content may be displayed on interlaced devices by displaying video lines p0, p1′, p2, p3′, p4, . . . in an even field interval ( 1/60 sec), and p0′, p1, p2′, p3, . . . in a next odd field interval ( 1/60 sec), where p1′, p3′, . . . are spatially interpolated lines from p0, p2, p4, . . . , and p0′, p2′, . . . are spatially interpolated lines from p1, p3, p5, . . . If “n” (number of progressive video lines in a progressive frame) is not equal to “m/2” (where m is the number of interlaced video lines in an interlaced frame), then vertical scaling is performed for each independent field using the lines from the even and odd fields as defined above. The original lines of the progressive frame are displayed in the correct relative vertical positions with respect to each other. The interpolated lines are positioned in between the original lines, using a different set of interpolated lines for each field as described above. However, the interpolation process causes a display artifact of vertical blur that reduces the quality of the original video.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with certain embodiments of the present invention as set forth in the remainder of the present application with reference to the drawings.